Look in to getting an international driving permit. Some countries require this. It took me 20 minutes to get mine from AAA in the United States.
Take photos of the vehicle before leaving the parking lot and GET FULL COVERAGE! The antenna of the vehicle I was driving, snapped off from the cold brisk wind. See photo below!
Get the GPS. Our cellphones died. It consumed more juice than the plug could provide in the car. With time constraints, an extra 30 minutes could be detrimental. Also guided tours are sometimes programmed with it!
Make sure there is at least $1,000.00 available on your credit card for deposits and incidents. When renting a car in Heathrow, we decided to get zero insurance at the office because we purchased it online. That resulted in a $1,000.00 deposit instead of $400.00 for a $20.00 coverage. We called the online company and canceled our insurance through them to buy it at the counter to reduce our deposit instead.
Pay everything with credit card. One, so it's tracked and if there are any disputes such as the rental company telling you to fill to a specific amount, you can file the claim with your credit card company. Two, I just learned this, some credit card companies also have insurance. Do some research online and let me know what you find! Three, this includes paying gas with credit card. Your card is protected by the FDIC, at least in the United States. If your card is scammed, there will be hope.
Know your region. Traveling through the land of fire and ice? Get a 4x4 or at least something that allows you to drive over a small stream. Travelling through the city? That Lexus is nice when your lanes are bigger, otherwise you'll be puckering pretty hard when cars are coming in your lane to get around parked vehicles, especially at 50 kilometers per hour. Also, know which side of the road you're driving on!
Watch out for speed cameras!!! Your GPS will sometimes help you.
Sign up for all rental companies. Their points and perks will add up. Only renting once? Call a friend up and give them your points!
You have to have a credit card in order to rent any vehicle. Make sure you have one.
Only use one person as the driver and make sure they're older than suggested age limits to avoid extra charges.
Fill up, fill up, fill up. Keep your car above half tank. Especially in an unknown region. The next station could be 20 miles away, which could be the doom of your holiday. Especially if you've been car camping overnight with the heat turned up to keep warm. Travelling is already stressful. There's no need to sweat over a low fuel light.
Before renting, read a blog or two to understand what might not be covered with insurance (like tires in Iceland!)
Also research to make sure you are not double paying insurances when buying traveler's insurance.
As I think of more, I'll update this article.
If there are any other great tips for renting a vehicle in another country, please post them below!